1. Field of the Invention
This invention relates to a war-absorbent resin allowing fast absorption without and under load, drying easily, and incurring only a small load during pulverization and a method for the production thereof. Still more particularly, it relates to a method for the production of a hydrophilic polymer allowing fast absorption, drying easily, and incurring only a small load during pulverization.
2. Description of the Prior Art
In recent years, water-absorbent resins capable of absorbing some tens to some hundreds of times their own weight of water have been developed. The water-absorbent resins of various types have been finding utility in applications requiring absorption and retention of water such as in the field of agriculture and horticulture, the field of fresh and perishable foodstuffs, and the field of industrial products requiring protection against dew formation and refrigeration as well as the field of hygienic materials including sanitary goods and disposable diapers.
As typical examples of the known water-absorbent resins, hydrolyzed starch-acrylonitrile graft polymer (JP-B-49-43,395), neutralized starch-acrylic acid graft polymer (JP-A-51-125,468), saponified vinyl acetate-acrylic acid copolymer (JP-A-52-14,689), hydrolyzed acrylonitrile copolymer or acrylamide copolymer (JP-B-53-15,959), cross-linked products thereof, self-cross-linked polysodium acrylate obtained by the reversed-phase suspension polymerization (JP-A-53-46,389), and partially neutralized cross-linked polyacrylic acid (JP-A-55-84,304) may be cited.
The quality that is required by a given water-absorbent resin varies with the kind of use for which the resin is intended. The properties which the water-absorbent resins intended for sanitary materials are expected to possess include high ratio of cubic expansion due to absorption under the load, high speed of absorption, and a marked ability to pass water, for example. These properties, however, do not always a positive correlation. It has been difficult to improve these properties all at once.
With a view to improving the absorbing speed of a given water-absorbent resin, attempts have been under way to enlarge the surface area of the water-absorbent resin by decreasing the particle diameter of the water-absorbent resin or molding the resin in the form of granules or scales. Generally, when the water-absorbent resin is molded in the form of particles of a small diameter, however, the water-absorbent resin particles, on contact with an aqueous liquid, form the so-called "wet clusters of powder" at a sacrifice of the absorbing speed. When the water-absorbent resin is subjected to size enlargement, the individual pellets of the product of size enlargement on contact with an aqueous liquid undergo the phenomenon of conversion into "wet clusters of powder" and rather suffer a decrease of the absorbing speed. When the water-absorbent resin is molded in the form of thin pieces, the produced thin pieces have no fully sufficient absorbing speed because this molding induces gel blocking in spite of the capability of improving the absorbing speed. Further, the formation of the water-absorbent resin in the form of thin pieces is uneconomical because the produced water-absorbent resin is inevitably so bulky as to require large facilities for transportation and storage.
Techniques for preventing the formation of wet clusters of powder and improving the absorbing speed by cross-linking the molecular chains in the surface region of the water-absorbent resin and increasing the cross-link density of the surface layer thereof have been proposed. These techniques have been disclosed in JP-A-57-44,627, JP-A-58-42,602, JP-B-6018,690, JP-A-58-180,233, JP-A-59-62,665, and JP-A-61-16,903, for example. These techniques have indeed improved the absorbing speed to a certain extent. The water-absorbent resins obtained by these techniques, however, are actually in such a state that minute particles having a smaller particle diameter than the optimum diameter aimed at by the techniques are contained in a considerable ratio. When the water-absorbent resin of this quality is used, it fails to show a fully satisfactory absorbing speed and incurs a decline in the ability to pass liquid in consequence of gel blocking.
The improvement of the absorbing speed by the use of a foaming agent, namely a method for pyrolytically forming bubbles in the water-absorbent resin, relies on the heat generated by polymerization to start the foaming has entailed such problems as (1) the polymerization in process entraining a large change in volume, allowing no easy control, and producing a polymer lacking homogeneity in quality and having a particularly high content of water-insoluble components and (2) the produced foam lacking stability of pore diameter and distribution and allowing now sufficient control of absorbing speed.
When this method is carried out by the use of an azo based polymerization initiator, the amount of this initiator must be increased to form bubbles in an amount enough to improve the absorbing speed and consequently the content of water-soluble components in the produced polymer tends to increase. Further, the method using the azo based polymerization initiator, similarly to the method using the foaming agent, has the problem that the polymerization in process will entrain a large change of volume and will consequently allow no easy control of pore particle and distribution of bubbles.
When this method implements the polymerization in the presence of dispersion of such a water-insoluble foaming agent as a volatile organic compound, though the polymerization can be effected relatively stability, the method incurs heavy waste of energy, proves expensive, and lacks practical serviceability because the polymerization requires a special apparatus from the viewpoint of safety on account of the use of the volatile organic compound and the used volatile organic compound is discharged from the system.
These water-absorbent resins, however, are invariably at a disadvantage in showing no sufficient absorbing speed in the without and under load, allowing no easy drying, suffering a large load during the pulverization, lacking uniformity of pore diameter, and having a large content of water-soluble components.
In contrast, the present invention has realized a water-absorbent resin which allows production of a foam uniform in pore diameter and distribution of bubbles, permits fast absorption of water without and under load, dries easily, and suffers only a small load during the pulverization and a method for the production of thereof.
An object of this invention, therefore, is to provide a water-absorbent resin capable of fast absorption of water and a method for the production thereof.
Another object of this invention is to provide a water-absorbent resin allowing fast absorption of water without and under load, drying easily, and suffering only small load during the pulverization and a method for the production thereof.
Still another object of this invention is to provide a water-absorbent resin having a large capacity for water absorption and having only a small content of water-soluble components.
A further object of this invention is to provide a method for the production of a hydrophilic polymer having bubbles dispersed therein.
Yet another object of this invention is to provide a method for the production of a hydrophilic polymer having a high water-absorbent speed or solving speed.
A still further object of this invention is to provide a water-absorbent resin which exhibits a very high ratio of cubic expansion due to water absorption under load in spite of a small particle diameter, possesses a large absorption speed due to the small particle diameter, and serves favorably for such hygienic materials as disposable diapers, sanitary napkins, and pads intended for patients of incontinence and a method for the production thereof.